Falling strand devolatilization has been employed for many years to separate a volatile component from a liquid component. Such devolatilization procedure has been especially useful in the manufacture of polymers, such as homopolystyrene where, as in a continuous mass polymerization process, a fluid mixture of homopolystyrene with styrene monomer results, which mixture must be subjected to a post polymerization treatment to remove the unreacted styrene monomre therefrom. For such a removal, a falling strand devolatilizer is a convenient piece of apparatus.
A falling strand devolatilizer as known to the prior art comprises as a preheater a shell and tube heat exchanger adapted to feed heated fluid material from the tubes thereof into a flash tank. An upper portion of a flash tank bears an outlet port which is typically interconnected with a source of vacuum. A fluid mixture being devolatilized is first subjected to a preheating step in the shell and tube heat exchanger after which the so heated mixture is subjected to a flashing step in which the so heated mixture is discharged into the flash tank wherein the conditions of pressure and temperature are so regulated that volatiles (e.g. especially monomer) are above their boiling point while the desired component (e.g. homopolystyrene) is maintained below its boiling or decomposition point. Vaporization of volatiles is prompted by continuously exhausting vapor of volatiles through the vacuum outlet port of the flash tank. Commonly as in the falling strand devolatilization of a polymer such as homopolytyrene admixed with styrene monomer, it is preferred to subject the polymer/monomer mixture to two successive stages of falling strand devolatilization, the conditions of temperature and pressure employed in the second stage being somewhat more rigorous than those used in the first stage. For each such stage of devolatilization, it has heretofore been thought necessary to employ a heat exchanger assembly preceding each of the two flash tanks.
It has been disclosed in U.S. Pat. No. 3,853,672 that in two stage falling strand devolatilization, the presence of a second preheater can be completely avoided between the first and second flash vaporization tanks. This discovery makes it possible to eliminate a costly sub-assembly heretofore thought necessary in two successive stages of falling strand devolatilization. In addition, this discovery permits the pressure to be varied in the first chamber without affecting the pressure or the resulting residual monomer in the second chamber. Regulation of this first chamber pressure can be used to control the residence time through the preheater, and the stripping effect of high boiling materials and the properties associated with the time/temperature history. Furthermore the elimination of a second heat exchanger can improve the properties of many polymeric materials by minimizing the time/temperature history.
To accomplish the elimination of the second heat exchanger, and, at the same time, maintain during devolatilizer operation an adequate pressure differential between the first flash zone and the second flash zone, it was found necessary to interpose between the first and the second flash vaporization tanks in place of the shell and tube heat exchanger, a variable fluid transfer regulation means such as a valve or a pump which is adapted to maintain a prechosen fluid material level in the bottom of the first flash vaporization tank.
Such a fluid level is used to maintain during apparatus operation a necessary and desirable pressure differential between the first and the second flash vaporization tanks and still permit fluid material to flow from the one into the second flash tank continuously.